Method for the reduction of NOx and NH3 emissions

ABSTRACT

This catalyst system simultaneously removes ammonia and enhances net NO x  conversion by placing an NH 3 —SCR catalyst formulation downstream of a lean NO x  trap. By doing so, the NH 3 —SCR catalyst adsorbs the ammonia from the upstream lean NO x  trap generated during the rich pulses. The stored ammonia then reacts with the NO x  emitted from the upstream lean NO x  trap—enhancing the net NO x  conversion rate significantly, while depleting the stored ammonia. By combining the lean NO x  trap with the NH 3 —SCR catalyst, the system allows for the reduction or elimination of NH 3  and NO x  slip, reduction in NO x  spikes and thus an improved net NO x  conversion during lean and rich operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/065,470, filed Oct. 22, 2002, which has issued as U.S. Pat. No. 7,332,135.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a catalyst system to facilitate the reduction of nitrogen oxides (NO_(x)) and ammonia from an exhaust gas. More particularly, the catalyst system of this invention includes a lean NO_(x) trap in combination with an ammonia selective catalytic reduction (NH₃—SCR) catalyst, which stores the ammonia formed in the lean NO_(x) trap during rich air/fuel operation and then reacts the stored ammonia with nitrogen oxides to improve NO_(x) conversion to nitrogen when the engine is operated under lean air/fuel ratios. In an alternate embodiment, a three-way catalyst is designed to produce desirable NH₃ emissions at stoichiometric conditions and thus reduce NO_(x) and NH₃ emissions.

2. Background Art

Catalysts have long been used in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NO_(x)) produced during engine operation into non-polluting gases such as carbon dioxide, water and nitrogen. As a result of increasingly stringent fuel economy and emissions standards for car and truck applications, it is preferable to operate an engine under lean conditions to improve vehicle fuel efficiency and lower CO₂ emissions. Lean conditions have air/fuel ratios greater than the stoichiometric ratio (an air/fuel ratio of 14.6), typically air/fuel ratios greater than 15. While lean operation improves fuel economy, operating under lean conditions increases the difficulty in treating some polluting gases, especially NO_(x).

Regarding NO_(x) reduction for diesel and lean burn gasoline engines in particular, lean NO_(x) adsorber (trap) technologies have been widely used to reduce exhaust gas NO_(x) emissions. Lean NO_(x) adsorbers operate in a cyclic fashion of lean and rich durations. The lean NO_(x) trap functions by adsorbing NO_(x) when the engine is running under lean conditions—until the NO_(x) trap reaches the effective storage limit—followed by NO_(x) reduction when the engine is running under rich conditions. Alternatively, NO_(x) reduction can proceed by simply injecting into the exhaust a sufficient amount of reductant that is independent of the engine operation. During this rich cycle, a short rich pulse of reductants, carbon monoxide, hydrogen and hydrocarbons reduces the NO_(x) adsorbed by the trap during the lean cycle. The reduction caused during the rich cycle purges the lean NO_(x) adsorber, and the lean NO_(x) adsorber is then immediately available for the next lean NO_(x) storage/rich NO_(x) reduction cycle. In general, poor NO_(x) reduction is observed if the air excess ratio λ is above 1. NO_(x) reduction generally increases over lean NO_(x) adsorbers as the λ ratio is decreased lower than 1. This air excess or lambda ratio is defined as the actual air/fuel ratio divided by the stoichiometric air/fuel ratio of the fuel used. The use of lean NO_(x) adsorber (trap) technology, and in particular the rich pulse of reductants, can cause the λ ratio to reach well below 1.

Lean NO_(x) traps, however, often have the problem of low NO_(x) conversion; that is, a high percentage of the NO_(x) slips through the trap as NO_(x). NO_(x) slip can occur either during the lean portion of the cycle or during the rich portion. The lean NO_(x) slip is often called “NO_(x) breakthrough”. It occurs during extended lean operation and is related to saturation of the NO_(x) trap capacity. The rich NO_(x) slip is often called a “NO_(x) spike”. It occurs during the short period in which the NO_(x) trap transitions from lean to rich and is related to the release of stored NO_(x) without reduction. Test results depicted in FIG. 1 a have shown that during this lean-rich transition, NO_(x) spikes, the large peaks of unreacted NO_(x), accounts for approximately 73% of the total NO_(x) emitted during the operation of a lean NO_(x) trap. NO_(x) breakthrough accounts for the remaining 27% of the NO_(x) emitted.

An additional problem with lean NO_(x) traps arises as a result of the generation of ammonia by the lean NO_(x) trap. As depicted in FIG. 1 b, ammonia is emitted into the atmosphere during rich pulses of the lean NO_(x) adsorber. In laboratory reactor experiments, ammonia spikes as high as 600 ppm have been observed under typical lean NO_(x) adsorber operation (see FIG. 1 b). While ammonia is currently not regulated, ammonia emissions are being closely monitored by the U.S. Environmental Protection Agency; and, therefore, reduction efforts must be underway. Ammonia is created when hydrogen or hydrogen bound to hydrocarbons reacts with NO_(x) over a precious metal, such as platinum. The potential for ammonia generation increases for a precious metal catalyst (such as a lean NO_(x) trap) as the λ ratio is decreased, as the duration of the rich pulse increases, and the temperature is decreased. There is thus an optimum lambda and rich pulse duration where the maximum NO_(x) reduction is observed without producing ammonia. Attempts to enhance conversion of NO_(x) by decreasing the λ ratio of the rich pulse duration leads to significant production of ammonia and thus results in high gross NO_(x) conversion (NO_(x)→N₂+NH₃), but much lower net NO_(x) conversion (NO_(x)→N₂).

In addition to nitrogen, a desirable non-polluting gas, and the undesirable NH₃ described above, N₂O is another NO_(x) reduction products. Like NH₃, N₂O is generated over NO_(x) adsorbers and emitted into the atmosphere during rich pulses. The gross NO_(x) conversion is the percent of NO_(x) that is reduced to N₂, N₂O and NH₃. The net NO_(x) conversion is the percent of NO_(x) that is reduced to nitrogen, N₂, only. Accordingly, the gross NO_(x) conversion is equal to the net NO_(x) conversion if nitrogen is the only reaction product. However, the net NO_(x) conversion is almost always lower than the gross NO_(x) conversion. Accordingly, a high gross NO_(x) conversion does not completely correlate with the high portion of NO_(x) that is reduced to nitrogen.

The NO_(x) conversion problem is magnified for diesel vehicles, which require more than a 90% NO_(x) conversion rate under the 2007 U.S. Tier II BIN 5 emissions standards at temperatures as low as 200° C. While high NO_(x) activity is possible at 200° C., it requires extreme measures such as shortening the lean time, lengthening the rich purge time, and invoking very rich air/fuel ratios. All three of these measures, however, result in the increased formation of NH₃ or ammonia. Accordingly, while it may be possible to achieve 90+% gross NO_(x) conversion at 200° C., to date there has not been a viable solution to achieve 90+% net NO_(x) conversion.

Accordingly, a need exists for a catalyst system that eliminates NO_(x) breakthrough during the lean operation as well has the NO_(x) spikes during the lean-rich transition period. There is also a need for a catalyst system that is capable of improving net NO_(x) conversion. Finally, there is a need for a catalyst system capable of reducing ammonia emissions.

SUMMARY OF THE INVENTION

This invention provides a solution for all of the above problems and, in particular, reduces or eliminates ammonia emissions and improves the net NO_(x) conversion of the catalyst system. These problems are solved by simultaneously removing ammonia and enhancing NO_(x) conversion with the use of an NH₃—SCR catalyst placed downstream of the lean NO_(x) adsorber catalyst, as shown in FIG. 2. The NH₃—SCR catalyst system serves to adsorb the ammonia emissions from the upstream lean NO_(x) adsorber catalyst generated during the rich pulses. Accordingly, as shown in FIG. 2, the ammonia emissions produced by the lean NO_(x) adsorber is stored and effectively controlled by the NH₃—SCR catalyst rather than being emitted. This reservoir of adsorbed ammonia then reacts directly with the NO_(x) emitted from the upstream lean NO_(x) adsorber. As a result, as shown in FIG. 3, the overall net NO_(x) conversion is enhanced from 55% to 80%, while depleting the stored ammonia, as a function of the SCR reaction: NH₃+NO_(x)→N₂. The NH₃—SCR catalyst is then replenished with ammonia by subsequent rich pulses over the lean NO_(x) adsorber.

During the lean cycle for this lean NO_(x) adsorber+NH₃—SCR system, the NO_(x) breakthrough from the upstream lean NO_(x) adsorber is reduced continuously as it passes over the NH₃—SCR until the reservoir of ammonia is depleted. In addition, during the rich cycle, large spikes of unreacted NO_(x) are created. The downstream NH₃—SCR catalyst thus serves to dampen these large NO_(x), spikes by reacting the unreacted NO_(x), with the reservoir of stored ammonia emitted from the lean NO_(x) adsorber. In general, the combination of the lean NO_(x), adsorber+NH₃—SCR catalyst system allows for the reduction, or elimination, of ammonia emissions and NO_(x) slip, i.e., reduction of NO_(x) breakthrough and NO_(x) spikes and, therefore, improved net NO_(x) conversion during lean and rich operation.

Additionally, under this invention, urea and/or ammonia does not need to be injected into the exhaust system to effectuate the reaction between NO_(x) and ammonia. Rather, the ammonia is automatically generated from the NO_(x) present in the exhaust gas as it passes over the precious metal lean NO_(x) adsorber during the rich pulses. The generated ammonia is then stored on the downstream NH₃—SCR catalyst, to react with the unreacted NO_(x), and thereby convert the unreacted NO_(x) to nitrogen.

The NH₃—SCR catalyst thus serves to adsorb the ammonia from the upstream lean NO_(x) adsorber catalyst generated during the rich pulses. Under this system, the ammonia is stored and effectively controlled rather than being emitted. This reservoir of adsorbed ammonia then reacts directly with any NO_(x) emitted from the upstream lean NO_(x) adsorber. As a result, the overall net NO_(x) conversion is enhanced from 55% to 80%, while the overall gross NO_(x) conversion is enhanced from 68% to 82%, as shown in FIG. 3.

In one alternative embodiment of this invention, the catalyst system can be optimized and NO_(x) reduction increased by vertically slicing the lean NO_(x) trap and NH₃—SCR catalyst substrates to create separate catalyst zones, such that the catalytic converter shell or can would have alternating sections of lean NO_(x) trap and NH₃—SCR catalysts, as shown in FIGS. 4 a, 4 b and 4 c. Under this embodiment, both technologies, the lean NO_(x) trap formulation and the NH₃—SCR formulation, can be incorporated into a single substrate and/or a single converter can rather than placing the NH₃—SCR catalyst downstream of the lean NO_(x) adsorber as two separate and distinct catalyst substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a graph illustrating the NO_(x) spikes that occur during the NO_(x) trap lean-rich transition;

FIG. 1 b is a graph illustrating NO_(x) and NH₃ emissions from a typical prior art lean NO_(x) adsorber system;

FIG. 2 depicts the lean NO_(x) and NH₃—SCR catalyst system of the present invention;

FIG. 3 depicts reduced NO_(x) emissions and NH₃ emissions as a result of the use of the lean NO_(x) and NH₃—SCR catalyst system of the present invention, as shown in FIG. 2;

FIGS. 4 a, 4 b, and 4 c depict three different zoned catalyst embodiments of the lean NO_(x) and NH₃—SCR catalyst system;

FIGS. 5 a, 5 b, and 5 c provide graphs illustrating the reduced levels of NO_(x) and NH₃ emissions resulting from each of the three zoned catalyst embodiments depicted in FIGS. 4 a, 4 b, and 4 c at a 250° C. inlet gas temperature and operating at a 50 second lean cycle and 5 second rich cycle;

FIGS. 6 a, 6 b and 6 c provide graphs illustrating the reduced levels of NO_(x) and NH₃ emissions resulting from each of the three zoned catalyst embodiments depicted in FIGS. 4 a, 4 b and 4 c at a 200° C. inlet gas temperature and operating at a 25 second lean cycle and a 5 second rich cycle;

FIGS. 7 a, 7 b and 7 c show three proposed examples of washcoat configurations incorporating the lean NO_(x) trap and NH₃—SCR formulations into the same substrate;

FIG. 8 is a graph illustrating the impact of NO_(x) conversion after hydrothermal aging; and

FIG. 9 depicts a modified three-way catalyst and NH₃—SCR catalyst system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In this invention, net NO_(x) conversion is improved and ammonia emissions reduced through the use of a lean NO_(x) trap and NH₃—SCR catalyst system which operate together to produce and store ammonia and reduce NO_(x) to nitrogen. In so doing, the catalyst system of the present invention solves three problems of lean NO_(x) traps; namely, reducing NO_(x) breakthrough, NO_(x) spikes and ammonia emissions.

In order to meet increasingly stringent fuel economy standards, it is preferable to operate an automotive engine under lean conditions. However, while there is improvement in fuel economy, operating under lean conditions has increased the difficulty in reducing NO_(x) emissions. As an example, for a traditional three-way catalyst, if the air/fuel ratio is lean even by a small amount, NO_(x) conversion drops to low levels. With traditional three-way catalysts, the air/fuel ratio must be controlled carefully at stoichiometric conditions to maximize reduction of hydrocarbons, carbon monoxide and NO_(x).

Throughout this specification, NO_(x) refers to nitrogen oxides, which include nitrogen monoxide NO and nitrogen dioxide NO₂. Further, lean NO_(x) adsorber and lean NO_(x) trap are used interchangeably throughout this specification.

To achieve NO_(x) reduction, under lean operating conditions, one option is the inclusion of a lean NO_(x) trap. While the lean NO_(x) trap is generally effective in NO_(x) reduction, lean NO_(x) traps are known to have the problems referred to as “NO_(x) slip” which includes breakthrough of NO_(x) during the extended lean operation of the NO_(x) trap and also NO_(x) spikes generated during the transition from the lean to the rich cycle.

NO_(x) spikes, or NO_(x) emissions during the lean-rich transition, are believed to occur due to the exothermic heat generated from the oxidation of reductants, carbon monoxide, hydrocarbons and hydrogen, by the oxygen released from the oxygen storage material—the temperature rise can be as high as 80-100° C.

The problem of NO_(x) spikes is illustrated in FIG. 1 a, and the problem of insufficient net NO_(x) conversion is illustrated in FIG. 1 b. FIG. 1 b depicts laboratory reactor data of a lean NO_(x) adsorber system operating in an 85 second lean and 5 second rich cyclic pattern. The plot in FIG. 1 b shows the nitrogen species concentration as a function of time. The laboratory reactor data depicted in FIG. 1 b resulted from a catalyst having an engine swept volume (ESV) of 100%. Additionally, the reactor used to obtain the results in FIG. 1 b was at a temperature of 300° C. To begin the cycle, 500 ppm of nitrogen oxide was fed into the reactor where much of it was stored during the 85 second lean duration. During the 5 second rich duration, nitrogen oxide was reduced; however, a significant amount of ammonia was formed. As illustrated in FIG. 1 b, the data shows ammonia spikes as high as 600 ppm under typical lean NO_(x) adsorber operation. Conversion, however, is generally improved as the λ ratio is decreased during the rich pulse. Decreasing the λ ratio also leads to significant production of ammonia and thus results in high gross NO_(x) conversion (NO_(x)→N₂+NH₃), but much lower net NO_(x) conversion (NO_(x)→N₂). As illustrated in FIG. 1 b, the net NO_(x) conversion to nitrogen for this lean NO_(x) adsorber system was only 55%.

Under the catalyst system of this invention, ammonia is reduced and the net NO_(x) conversion improved simultaneously by placing an NH₃—SCR catalyst formulation downstream of the lean NO_(x) adsorber catalyst, as shown in FIG. 2.

FIG. 2 is an illustration of the catalyst system of this invention, which is capable of simultaneously eliminating ammonia emissions and improving net NO_(x) conversion. As illustrated in FIG. 2, NO_(x) produced during engine operation is stored by the lean NO_(x) adsorber during the lean cycle. Following the lean cycle, during the rich cycle of the lean NO_(x) adsorber, NO_(x) is reduced and ammonia generated. The lean NO_(x) adsorber stores much of the NO_(x) during the lean operation and then reduces NO_(x) during rich pulses of the reductants. During the same rich pulses, significant amounts of ammonia are generated, as further illustrated in FIG. 1. As illustrated in FIG. 2, the lean NO_(x) adsorber emits NO, NO₂, NH₃, and N₂O. These same gases then pass through the NH₃—SCR, where NH₃ is stored. Accordingly, the addition of the NH₃—SCR catalyst downstream allows for the adsorption of NH₃ and subsequent reaction with any NO_(x) that slips through the upstream lean NO_(x) adsorber, which thus improves the overall net NO_(x) conversion (NH₃+NO→N₂). As can be seen in FIG. 2, the catalyst system of this invention results in a significant net NO_(x) conversion improvement, the elimination of ammonia emissions, and the production of non-polluting gases nitrogen and N₂O.

It should be noted that for diesel applications, lean NO_(x) adsorbers must operate at lower temperatures compared to gasoline lean NO_(x) adsorbers since the exhaust temperatures of diesel engines are significantly lower. More ammonia is generated at 200° C. than at 300° C. over lean NO_(x) adsorbers, and thus the catalyst system of this invention has an even greater potential for diesel applications. Likewise, the problem of NO_(x) spikes is more critical at higher temperatures, the temperatures used for gasoline applications; and thus the catalyst system of this invention is beneficial to control the unreacted NO_(x) spikes that result from the operation of a lean NO_(x) adsorber at operating temperatures typical for gasoline lean NO_(x) adsorber applications.

The NH₃—SCR catalyst thus serves to adsorb the ammonia produced naturally from the upstream lean NO_(x) adsorber catalyst generated during the rich pulses. As a result, the NH₃—SCR catalyst stores the ammonia, controlling it rather than allowing it to be emitted into the atmosphere. This reservoir of adsorbed NH₃ in the NH₃—SCR catalyst reacts directly with the NO_(x) emitted from the upstream lean NO_(x) adsorber (trap).

In general, this invention works to clean NO_(x) emissions—and thus has applicability for stationary sources as well as for moving vehicles. This invention may be used to reduce NO_(x) emissions for nitric acid plants, or any other stationary source that requires the reduction of NO_(x) emissions. This invention is nonetheless particularly directed for use with gasoline and diesel vehicles which, unlike stationary sources, have a wide range of operating parameters, especially temperature parameters—which cannot be precisely controlled. The present invention has the ability to store large quantities of ammonia across a broad temperature range to effectuate the reaction between ammonia and nitrogen oxides and thereby convert NO_(x) to nitrogen.

As illustrated in FIG. 3, laboratory experiments have demonstrated that the use of a lean NO_(x) adsorber plus NH₃—SCR catalyst system improves net NO_(x) conversion from 55%, as illustrated in FIG. 1, to 80%. FIG. 3 is a graph displaying laboratory data obtained using the catalyst system of this invention, wherein NO_(x) ppm are charted as a function of time. As illustrated in FIG. 3, the catalyst system of this invention completely eliminated the ammonia spikes created during the rich pulses of the lean NO_(x) adsorber. In this system, ammonia is stored on the NH₃—SCR catalyst where it reacts with NO_(x) during the 85 second lean duration, which thus improves the net NO_(x) conversion from 55% to 80% with no additional fuel economy penalty. As shown in FIG. 3, the improved net NO_(x) conversion can be observed by the much narrower profile—zero ppm NO_(x) is emitted for a significant amount of time as compared to the graph shown in FIG. 1 of a system lacking the NH₃—SCR+lean NO_(x) adsorber combination.

The reaction between the stored ammonia and NO_(x) increases the overall net NO_(x) conversion, which is enhanced from 55%—the amount of NO_(x) converted in prior art lean NO_(x) trap systems—to 80%—as a result of the combination of a lean NO_(x) trap and NH₃—SCR catalyst system. Moreover, in addition to improving net NO_(x) conversion, the ammonia stored in the NH₃—SCR catalyst is depleted during the SCR reaction wherein ammonia and nitrogen oxide are reacted to produce nitrogen. The NH₃—SCR catalyst is replenished with ammonia by subsequent rich pulses over the lean NO_(x) adsorber that causes a portion of the NO_(x) to react with hydrogen to form ammonia.

It should be noted that no urea or ammonia needs to be injected into the exhaust system to effectuate the reaction between ammonia and NO_(x). Rather, the ammonia is naturally generated from the NO_(x) present in the exhaust gas as it passes over the lean NO_(x) trap during rich pulses. More specifically, ammonia is naturally created during the fuel rich cycle of the lean NO_(x) trap. Ammonia is naturally produced as it passes over the precious metal active component of the lean NO_(x) trap. Similarly, the ammonia could also be generated in a conventional precious metal based TWC located upstream of a LNT/NH₃—SCR system.

For this invention, the lean NO_(x) trap is optimized for ammonia generation by removing oxygen storage capacity (OSC) and thereby enhancing the rich cycle, and thus creating a greater quantity of ammonia for reaction with the NO_(x) in the downstream NH₃—SCR catalyst. In a preferred embodiment, the lean NO_(x) trap includes platinum as the precious metal. Platinum is the preferred precious metal because it is believed that a greater quantity of NH₃ is produced over platinum than rhodium, palladium and/or a combination of the precious metals. Nonetheless, other precious metals such as palladium and rhodium, and the combination of one or more of the precious metals platinum, palladium and rhodium may also be used to generate NH₃.

Additionally, the lean NO_(x) trap of this invention preferably includes a “NO_(x) adsorbing material” or NO_(x) storage component/material, which can be alkali and alkali earth metals such as barium, cesium, and/or rare earth metals such as cerium and/or a composite of cerium and zirconium. Although an alternative catalyst formulation that does not contain a NO_(x) storage component but generates ammonia from NO_(x) may also be utilized, in the most preferred embodiment, the NO_(x) storage material should have the ability to store NO_(x) at low temperature ranges, specifically in the range of 150° C.-300° C. The NH₃ thermodynamic equilibrium under rich conditions is maximized during the temperature range of 150° C.-300° C.

In general, to increase the NO_(x) storage function of the lean NO_(x) trap and effectuate the NO_(x) conversion reaction, in the preferred embodiment, the lean NO_(x) trap has the following characteristics: (1) the inclusion of platinum as the precious metal; (2) the ability to store NO_(x) between 150° C. and 500° C. during the lean portion of the cycle; (3) the ability to maximize the duration of the lean NO_(x) trap rich cycle; (4) the ability to generate ammonia at the 150° C.-500° C. temperature range; (5) minimize OSC to lessen fuel penalty; and (6) lower lambda to generate more ammonia. Ammonia production is maximized at the preferred temperature range, 150° C.-300° C.—which also correlates with the steady state equilibrium range for ammonia creation. It bears emphasis that other NO_(x) storage components may be utilized, especially for stationary sources, where sulfur poisoning does not pose a threat.

Most simply, the NH₃—SCR catalyst may consist of any material or combination of materials that can adsorb ammonia and facilitate the NO_(x)+NH₃ to yield nitrogen. The NH₃—SCR catalyst should preferably be made of a base metal catalyst on a high surface area support such as alumina, silica, titania, zeolite or a combination of these. More preferably, the NH₃—SCR catalyst should be made of a base metal selected from the group consisting of Cu, Fe and Ce and/or a combination of these metals, although other base metals may be used. Base metals generally are able to effectuate NO_(x), conversion using ammonia while both the base metals and the high surface support material serves to store NH₃. The base metal and high surface area support such as zeolite selected should preferably be one that can store NH₃ over the widest possible temperature range. Likewise, the base metal selected is preferably one that can convert NO and NO₂ to N₂ across the widest possible temperature range and the widest range of NO/NO₂ ratios.

The advantage of the catalyst system of this invention is the use of a combination of a lean NO_(x) trap and an NH₃—SCR catalyst. The use of a lean NO_(x) trap in the present system allows for much greater storage of NO_(x), because the NO_(x) breakthrough that would otherwise happen can be controlled by the NH₃—SCR catalyst. Additionally, the use of a lean NO_(x) trap as part of this system allows for the operation of the engine at lean conditions for a longer time, and thus provides improved fuel economy. If, for example, a three-way catalyst is used as the NO_(x) storage mechanism, NO_(x) storage is significantly limited, as well as the production of ammonia. To maximize the reduction of emissions, a three-way catalyst must be operated at stoichiometric conditions. Accordingly, unless the three-way catalyst is run on the rich side 100% of the time, ammonia production is significantly less than for a typical lean NO_(x) trap. As set forth above, the efficiency of a three-way catalyst is compromised if it is operated at conditions other than at stoichiometric conditions. Thus the combination of a lean NO_(x) trap and NH₃—SCR catalyst allows for significant NO_(x) storage and ammonia production and thus increases net NO_(x) conversion.

In a preferred embodiment, the lean NO_(x) trap and NH₃—SCR catalyst constitute alternating zones in a single substrate and/or a single catalytic converter can. This zoned design, as shown in three different embodiments in FIGS. 4 a-4 c, is believed to maximize the reaction between ammonia and NO_(x).

As illustrated in FIG. 4, three zoned catalyst system embodiments were evaluated on a laboratory flow reactor. The total catalyst system dimensions were held constant at a 1″ diameter and 2″ length. The first system, labeled “4 a”, had a 1″ long lean NO_(x) trap followed by a 1″ long NH₃—SCR catalyst. In the second system, labeled “4 b”, the catalyst samples were sliced in half to yield alternating ½″ long sections. Finally, in the third system, labeled “4 c”, the same catalyst samples were further cut in half to yield ¼″ long sections, again of the lean NO_(x) trap and NH₃—SCR catalyst technologies. It should be noted that each time the catalysts were sliced, as shown in “4 b” and “4 c”, the overall length of the catalyst system was reduced slightly, approximately 3/16″ total. The alternating lean NO_(x) trap and NH₃—SCR catalyst zones can be created in a single substrate or the lean NO_(x) trap and NH₃—SCR catalyst prepared, cut as desired and then placed adjacent one another in a single can. The zones are preferably formed in a single substrate. However, cut substrates placed in alternating fashion also exhibit improved net NO_(x) conversion.

Under the zoned catalyst designs shown in FIGS. 4 a-4 c, where alternating lean NO_(x) and NH₃—SCR catalyst zones are provided, the ammonia formed by the lean NO_(x) trap is believed to be immediately adsorbed by the NH₃—SCR catalyst for use in the NO_(x) conversion reaction. It is further believed that the greater the separation between the lean NO_(x) trap and the NH₃—SCR catalyst, the greater chance there is for the ammonia to be converted back into NO_(x). It is further believed that oxygen is more abundant in the back of a catalyst substrate and thus the oxygen may be available to effectuate the unwanted conversion of the ammonia back to nitrogen oxide. Accordingly, if the catalyst substrate is too long, there may be some undesired conversion that takes place; and thus in a preferred embodiment, the substrate is designed so that ammonia is available for immediate reaction with NO_(x).

FIGS. 5 a-5 c illustrate laboratory reactor data of the three different zoned catalyst system embodiments shown in FIGS. 4 a-4 c. This laboratory data was obtained with the three catalyst systems operating at a 250° C. inlet gas temperature and operating with 50 second lean and 5 second rich cycles. Additionally, the inlet concentration of the NO_(x) feed gas was 500 ppm and the overall space velocity was 15,000 per hour. As illustrated in FIGS. 5 a-5 c, with the use of a two-zoned catalyst system as depicted in FIG. 5 a, approximately 50 ppm of NO is emitted. This two-zone catalyst system resulted in a gross NO_(x) conversion of 95% and a net NO_(x) conversion of 66%. The four-zone catalyst embodiment, depicted as FIG. 5 b, significantly reduced NO_(x) emissions, well below the 15 ppm range, to result in gross NO_(x) conversion of 99% and a net NO_(x) conversion of 86%. Finally, as illustrated by the eight zoned catalyst embodiment, FIG. 5 c, gross NO_(x) conversion is 100% and net NO_(x) conversion is 97.5%. The improvement comes from the reduction of N₂O, elimination of the NH₃ breakthrough and reduction of NO_(x). Accordingly, as the catalyst system is zoned down from 1″ sections to ¼″ sections, the test results revealed an associated improvement in net NO_(x) conversion.

As shown in FIGS. 5 a-5 c, a zoned catalyst, with alternating lean NO_(x) and NH₃—SCR catalysts in 1″ to ¼″ sections significantly improves the net NO_(x) conversion from 66% to 97.5%. In addition, the gross NO_(x) conversion is improved from 95% to 100%. In general, the improvement in the net NO_(x) conversion is the function of the elimination of the ammonia slip, reduction in N₂O, and extra NO_(x) reduction related to the NH₃+NO_(x) reaction on the NH₃—SCR catalyst. It is further believed that the drop in N₂O emissions is likely due to a higher fraction of the NO_(x) reduction reaction proceeding on the NH₃—SCR catalyst rather than the lean NO_(x) trap. NO_(x) reduction over a platinum-containing-lean NO_(x) trap results in high levels of N₂O generation, whereas the NH₃—SCR catalyst has a high selectivity to nitrogen.

FIGS. 6 a-6 c depicts laboratory data obtained using the three-zoned catalyst embodiments originally shown in FIGS. 4 a-4 c at a 200° C. inlet gas temperature operating with a 25 second lean cycle and a 5 second rich cycle. As compared to FIGS. 5 a-5 c, it should be noted that shortening the lean time from 50 seconds, as used in FIGS. 5 a-5 c, to 25 seconds, resulted in a substantial higher steady emission of ammonia—a fact which results in reduced net NO_(x) conversion rates, as compared to the data charted in FIGS. 5 a-5 c. As can be seen in FIGS. 6 a-6 c, the use of smaller zoned sections from two zones to eight zones, and thus 1″ sections down to ¼″ sections, as illustrated in FIGS. 6 a and 6 c, improves the net NO_(x) conversion from 50% to 81%. Again, this improvement is believed to come mainly from the reduction of ammonia breakthrough and a small reduction in N₂O emissions. This lab data was obtained with an inlet concentration of the NO_(x) feed gas at 500 ppm and an overall space velocity at 15,000 per hour.

As set forth above, in the preferred embodiment, the lean NO_(x) trap washcoat and NH₃—SCR washcoat are combined in a single substrate rather than placing the NH₃—SCR formulation downstream of the lean NO_(x) adsorber as two separate catalyst substrates. Under this embodiment, the catalyst formulations can be incorporated together by mixing or layering the washcoats on a substrate.

FIGS. 7 a-7 c show three proposed washcoat configurations incorporating the lean NO_(x) trap and NH₃—SCR formulations into the same substrate. As shown in FIGS. 7 a and 7 b, the first and second proposed configurations have the lean NO_(x) trap and NH₃—SCR washcoat formulations on the bottom and top layer, respectively. It is believed that the top layer could be a highly porous structure that allows better and faster contact between the chemicals and gas phase and the active sites in the second layer. The third configuration, as shown in FIG. 7 c, involves the use of a one layer washcoat containing both lean NO_(x) trap and NH₃—SCR washcoat formulations. Under this third configuration, shown in FIG. 7 c, the washcoat composition of the lean NO_(x) trap and NH₃—SCR catalyst could be homogeneously or heterogeneously mixed. For a heterogeneously mixed composition, the formulation of the lean NO_(x) trap and NH₃—SCR catalyst are separated. However, they contact each other in varying degrees by controlling the size of the grain structures. The homogeneously mixed composition allows for a more intimate contact between the two formulations and is thus preferred.

The invention also contemplates engineering such combinations within the pores of the monolithic substrate. An example of this is incorporating washcoat into porous substrates used for filtering diesel particulate matter. Thus, this lean NO_(x) trap/NH₃—SCR catalyst concept can be integrated into diesel particulate matter devices.

This very active SCR reaction of NO_(x) and ammonia proceeds with or without oxygen present. Koebel et al. reports that the fastest SCR reaction involves equal molar amounts of NO and NO₂. NO and NO₂ then react with two NH₃ to yield N₂ in the absence of oxygen. In contrast, the lean NO_(x) adsorber reaction of NO_(x) plus CO is highly reactive only in an oxygen-free environment. In a lean NO_(x) adsorber system, NO_(x) is adsorbed during the lean cycle duration, NO_(x) is not reduced. Accordingly, NO_(x) reduction is limited to only the rich pulse time duration. On the other hand, the lean NO_(x) adsorber+NH₃—SCR catalyst system allows for NO_(x) reduction reaction to proceed during both the lean and rich time durations. Accordingly, ammonia as a reductant can be considered as a much more robust reductant than carbon monoxide.

As set forth above, the fastest SCR reaction involves equal molar amounts of NO and NO₂. Accordingly, FIG. 8 illustrates the impact of varying NO:NO₂ ratios after hydrothermal aging. FIG. 8 is a graph of three NH₃—SCR catalyst formulations over a wide NO:NO₂ range. In the laboratory, it was possible to control the NO:NO₂ ratio entering the downstream NH₃—SCR catalyst. Accordingly, the NO:NO₂ ratio entering the NH₃—SCR catalyst was solely dependent on the upstream lean NO_(x) adsorber. In some cases, the majority of the feed NO_(x) (especially NO_(x) spikes) are made up of mostly NO rather than NO₂. Accordingly, it is believed that the catalyst formulations of this invention will enhance reported net NO_(x) efficiency—and thus the preferred catalyst is one that is capable of operating across the broadest range of NO:NO₂ ratios, and at a full spectrum of temperature ranges.

In general, since NH₃—SCR catalysts do not contain precious metals, they are significantly less costly than a typical lean NO_(x) trap. Accordingly, it is more cost effective to have an overall catalyst system containing a lean NO_(x) trap adsorber and an NH₃—SCR catalyst system, rather than one that uses two lean NO_(x) trap adsorbers. Additionally, the incorporation of both a lean NO_(x) trap and NH₃—SCR washcoat into a single substrate will significantly reduce substrate costs.

In another embodiment of this invention, NH₃ and NO_(x) in an exhaust stream are reduced using a stoichiometric three-way catalyst system. This three-way catalyst system has particular application for high speed/high flow rate conditions (i.e., UOS6 conditions). Currently, three three-way catalysts are used for such high speed condition applications, wherein the third three-way catalyst is primarily directed to NO_(x) removal for high speed/high flow rate conditions. Under this alternate embodiment, the third three-way catalyst can be substituted with an NH₃—SCR catalyst to store NH₃ for reaction with NO_(x) to improve net NO_(x) conversion, eliminate NH₃ emissions and reduce catalyst costs.

To improve net NO_(x) and NH₃ reduction, the second three-way catalyst can be modified to enhance the three-way catalyst's ability to generate NH₃ emissions. To this end, in a preferred embodiment, the three-way catalyst is designed to generate desirable NH₃ creation by using platinum as the precious metal of the three-way catalyst, by placing platinum on the outer layer of the three-way catalyst to maximize the NO+H₂→NH₃ reaction. Likewise, the oxygen storage capacity (OSC) of the three-way catalyst can be removed to further promote the creation of “desirable” NH₃. By doing so, the NH₃ purposely generated during rich operation can then be stored by the NH₃—SCR catalyst for subsequent reaction with NO_(x) emissions, and thereby control both NO_(x) and NH₃ emissions under all operating conditions.

When a car is operated under rich conditions, the air/fuel ratio is less than 14.6, hydrogen is produced in the exhaust via the water-gas shift reaction: CO+H₂O→CO₂+H₂. The hydrogen that is produced then reacts with NO_(x) as it passes over the precious metal surface to create “desirable” ammonia. The ammonia produced is then stored on an NH₃—SCR catalyst to help reduce net NO_(x) conversion. The reaction of NO_(x)+NH₃→N₂+H₂O can then take place on a separate NH₃ selective catalyst, capable of converting NO₂ and NO to N₂.

As shown in FIG. 9, a stoichiometric three-way catalyst/NH₃—SCR catalyst system 10 is depicted, including a first three-way catalyst 14 positioned in close proximity to the engine 12 to reduce cold start emissions. The second three-way catalyst 16 is modified as described above to enhance the ability of the second three-way catalyst 16 to generate NH₃ emissions. Downstream of the second three-way catalyst 16 is an NH₃—SCR catalyst 18 that functions to store NH₃ produced by the modified second three-way catalyst 16 for reaction with NO_(x) emissions, to reduce both NO_(x) and NH₃ emissions.

By substituting the third three-way catalyst as currently used with an NH₃—SCR catalyst and thereby eliminating the need for a third precious metal containing catalyst, significant cost savings can be achieved.

It should further be noted that this invention also contemplates the use of a three-way catalyst, in combination with a lean NO_(x) trap and an NH₃—SCR catalyst.

While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims. 

1. A method of purifying exhaust gas of an internal combustion engine, the method comprising: generating ammonia in an exhaust system of said engine by a reaction of NO_(x) and reducing components in exhaust gases when the exhaust gases are in a reducing state; and removing NO_(x) downstream of the generated ammonia by adsorbing NO_(x) in the exhaust gases when the exhaust gases are in an oxidizing state, and reducing the adsorbed NO_(x) to generate ammonia and retaining the generated ammonia when the exhaust gases are in the reducing state.
 2. An exhaust gas purifying apparatus for an internal combustion engine, comprising: ammonia generating device, provided in an exhaust system of said engine, for generating ammonia by a reaction of NO_(x) and reducing components in exhaust gases when the exhaust gases are in a reducing state; and NO_(x) removing device, provided downstream of said ammonia-generating device, for adsorbing NO_(x) in the exhaust gases when the exhaust gases are in an oxidizing state, said NO_(x) removing device reducing the adsorbed NO_(x) to generate ammonia and retaining the generated ammonia when the exhaust gases are in the reducing state. 